1. Field of the Invention
The invention disclosed herein relates to geologic exploration and, in particular, to techniques for determining seismic events.
2. Description of the Related Art
Subterranean formations may be monitored using one or more seismic receivers. The receivers may be geophones placed at the surface or submerged in wells or on the ocean floor. Also, the receivers may be hydrophones placed in those same locations, but sensitive to only certain types of waves. The receivers placed in wells may be shallow (usually above the formation of interest) or deep (usually at or below the formation of interest). Seismic receivers may be sensitive to seismic waves along a certain axis or those traveling on any axis. Likewise, the receivers may be sensitive to only certain types of seismic waves, or several types. Those sensitive to certain axis of travel, called directional receivers, may be coupled with other directional receivers. For example, a directional receiver may be coupled with two other directional receivers in a set of three orthogonal receivers which collect information about the waves in three dimensions. This three-dimensional information may be rotated mathematically through the use of trigonometric functions in order to derive information as to wave travel in the x-axis, y-axis and z-axis relative to gravity. Alternatively, mathematical rotation may provide translation of the data relative to a wellbore, a cardinal direction, or any other reference point.
Microseismic monitoring concerns passively monitoring a formation for seismic events which are very small. Such events may include the seismic effects generated in a formation by fracturing, depletion, flooding, treatment, fault movement, collapse, water brealtbrough, compaction or other similar subterranean interventions or effects. One of the main problems with microseismic monitoring, as with other forms of seismic monitoring, is that of noise. With microseismic events, however, the problem is emphasized because the signal strength is generally very small. This means, in turn, that a small amount of noise which would not cause any significant effect as to a regular, active seismic survey causes a significant degradation of the signal to noise ratio in the microseismic survey.
The geology of the microseismic environment is also of interest. Different geological layers are composed of different materials which transmit seismic waves at different velocities. It will be appreciated that when a source occurs in a high-velocity layer, its transmission through to a lower-velocity layer will cause attenuation, as much of the wave energy is reflected back into the high-velocity layer.
What are needed are systems and methods for identifying and determining locations of seismic events, including microseismic events, and for automatic identification of such events.